Alan Aitken

The subglacial geology of Wilkes Land, East Antarctica

Wilkes Land is a key region for studying the configuration of Gondwana and for appreciating the role of tectonic boundary conditions on East Antarctic Ice Sheet (EAIS) behavior. Despite this importance, it remains one of the largest regions on Earth where we lack a basic knowledge of geology. New magnetic, gravity, and subglacial topography data allow the regions first comprehensive geological interpretation. We map lithospheric domains and their bounding faults, including the suture between Indo-Antarctica and Australo-Antarctica. Furthermore, we image subglacial sedimentary basins, including the Aurora and Knox Subglacial Basins and the previously unknown Sabrina Subglacial Basin. Commonality of structure in magnetic, gravity, and topography data suggest that pre-EAIS tectonic features are a primary control on subglacial topography. The preservation of this relationship after glaciation suggests that these tectonic features provide topographic and basal boundary conditions that have strongly influenced the structure and evolution of the EAIS.

Crustal architecture of the Capricorn Orogen, Western Australia and associated metallogeny

A 581 km vibroseis-source, deep seismic reflection survey was acquired through the Capricorn Orogen of Western Australia and, for the first time, provides an unprecedented view of the deep crustal architecture of the West Australian Craton. The survey has imaged three principal suture zones, as well as several other lithospheric-scale faults. The suture zones separate four seismically distinct tectonic blocks, which include the Pilbara Craton, the Bandee Seismic Province (a previously unrecognised tectonic block), the Glenburgh Terrane of the Gascoyne Province and the Narryer Terrane of the Yilgarn Craton. In the upper crust, the survey imaged numerous Proterozoic granite batholiths as well as the architecture of the Mesoproterozoic Edmund and Collier basins. These features were formed during the punctuated reworking of the craton by the reactivation of the major crustal structures. The location and setting of gold, base metal and rare earth element deposits across the orogen are closely linked to the major lithospheric-scale structures, highlighting their importance to fluid flow within mineral systems by the transport of fluid and energy direct from the mantle into the upper crust.

The architecture, kinematics, and lithospheric processes of a compressional intraplate orogen occurring under Gondwana assembly: The Petermann orogeny, central Australia

We ally aeromagnetic interpretation with constrained three-dimensional (3D) gravity inversion over the Musgrave Province in central Australia to produce a 3D architectural and kinematic model of the ca. 550 Ma compressional intraplate Petermann orogeny. Our model is consistent with structural, metamorphic, and geochronological constraints and crustal-scale seismic models. Aeromagnetic interpretation indicates that divergent thrusts at the margins of the province are cut by transpressional shear zones that run along the axis of the orogen. Gravity inversion indicates that the marginal thrusts are crustal-scale and shallow-dipping, but that the transpressional shear zones of the axial zone are more steeply dipping, and penetrate the crust-mantle boundary, accommodating offsets of 10–25 km. This thick wedge of mantle within the lower crust has been in isostatic disequilibrium for more than 500 Ma, and we suggest that this load may be supported by local lithospheric strengthening resulting from the emplacement of relatively strong lithospheric mantle within the relatively weak lower crust. Other orogenic processes inferred from the model include: probable inversion of relict extensional architecture; crustal-scale strain partitioning leading to strain accommodation by the vertical and lateral extrusion of relatively undeformed crustal blocks; and escape tectonics directed toward the relatively free eastern margin of the orogen. These processes are consistent with the concept that mechanical and thermal heterogeneities in the lithosphere, and the resulting feedbacks with deformation, are the dominant controls on intraplate orogenesis. This model also demonstrates that the architecture and kinematics of the Petermann orogeny require modifi cation of leading models of Gondwana assembly.

Repeated large-scale retreat and advance of Totten Glacier indicated by inland bed erosion

Climate variations cause ice sheets to retreat and advance, raising or lowering sea level by metres to decametres. The basic relationship is unambiguous, but the timing, magnitude and sources of sea-level change remain unclear; in particular, the contribution of the East Antarctic Ice Sheet (EAIS) is ill defined, restricting our appreciation of potential future change. Several lines of evidence suggest possible collapse of the Totten Glacier into interior basins during past warm periods, most notably the Pliocene epoch, causing several metres of sea-level rise. However, the structure and long-term evolution of the ice sheet in this region have been understood insufficiently to constrain past ice-sheet extents. Here we show that deep ice-sheet erosion—enough to expose basement rocks—has occurred in two regions: the head of the Totten Glacier, within 150 kilometres of today’s grounding line; and deep within the Sabrina Subglacial Basin, 350–550 kilometres from this grounding line. Our results, based on ICECAP aerogeophysical data, demarcate the marginal zones of two distinct quasi-stable EAIS configurations, corresponding to the ‘modern-scale’ ice sheet (with a marginal zone near the present ice-sheet margin) and the retreated ice sheet (with the marginal zone located far inland). The transitional region of 200–250 kilometres in width is less eroded, suggesting shorter-lived exposure to eroding conditions during repeated retreat–advance events, which are probably driven by ocean-forced instabilities. Representative ice-sheet models indicate that the global sea-level increase resulting from retreat in this sector can be up to 0.9 metres in the modern-scale configuration, and exceeds 2 metres in the retreated configuration.

Palaeoproterozoic accretion processes of Australia and comparisons with Laurentia

The Palaeoproterozoic rocks of Australia and Laurentia preserve an excellent record of the accretionary tectonics associated with transitions between the Columbia (Nuna) and Rodinia supercontinents. The geologic records of Australia and Laurentia suggest that the dominant tectonic driver was one or more subduction zones in which several episodes of crustal accretion occurred between ca. 1790 Ma and 1620 Ma. Correlated orogenic events include the ca. 1800–1780 Ma Yapungku–Yambah (Australia)–Medicine Bow (Laurentia) orogenies, ca. 1740–1690 Ma Strangways–Kimban (Australia)–Nimrod (Antarctica)–Yavapai (Laurentia) orogenies, and the ca. 1650–1620 Ma Leibig–Ooldea (Australia)–Mazatzal (Laurentia) orogenies. There are major differences in the style of accretion: Laurentia is characterized by accretion of dominantly juvenile arc terranes, whereas accreted Australian terranes are more evolved and are isotopically similar to the continental nucleus. Adjacent to its plate margin, the Australian continent contained regions of elevated heat production compared with the Laurentian margin. Brace-Goetze lithospheric strength models for ca. 1700 Ma indicate that the Australian plate margin was significantly weaker than that of Laurentia. This variation in lithospheric strength is interpreted to impact the behaviour of the overriding plate during subduction roll back. Attenuation of the Australian lithosphere during ductile extension caused rifting of large continental fragments from the plate margin. Their subsequent accretion resulted in the re-amalgamation of pre-existing continental lithosphere similar to the lithosphere in the overriding plate. Subduction roll back adjacent to the cold and rigid Laurentian margin (e.g. the Wyoming Craton) had relatively little impact on the overriding plate, with oceanic back-arc basins and juvenile arc terranes developing outboard of the plate margin. Inversion of these arc and oceanic back-arc terranes resulted in episodic continental growth.

Australia and Nuna

Abstract The Australian continent records c. 1860–1800 Ma orogenesis associated with rapid accretion of several ribbon micro-continents along the southern and eastern margins of the proto-North Australian Craton during Nuna assembly. The boundaries of these accreted micro-continents are imaged in crustal-scale seismic reflection data, and regional gravity and aeromagnetic datasets. Continental growth (c. 1860–1850 Ma) along the southern margin of the proto-North Australian Craton is recorded by the accretion of a micro-continent that included the Aileron Terrane (northern Arunta Inlier) and the Gawler Craton. Eastward growth of the North Australian Craton occurred during the accretion of the Numil Terrane and the Abingdon Seismic Province, which forms part of a broader zone of collision between the northwestern margins of Laurentia and the proto-North Australian Craton. The Tickalara Arc initially accreted with the Kimberley Craton at c. 1850 Ma and together these collided with the proto-North Australian Craton at c. 1820 Ma. Collision between the West Australian Craton and the proto-North Australian Craton at c. 1790–1760 Ma terminated the rapid growth of the Australian continent.

Assessing uncertainty in the integration of aeromagnetic data and structural observations in the Deering Hills region of the Musgrave Province

Integrated aeromagnetic and field-based structural analysis provides a method for establishing a constrained regional-scale tectonic model where insufficient outcrop, remoteness or restricted access precludes widespread structural mapping. In this method the different scales of observation lead to uncertainty in integrating the observations. An integrated analysis of the Deering Hills region of the Musgrave Province shows that this uncertainty is largely dependent on the magnetic data resolution, the scale of deformation, and mineralogy. In the Deering Hills, four deformation events are defined, each with different structural character. These differences in character result in different levels of uncertainty in integrating these observations with aeromagnetic data. D1 was only evident at small scales in outcrop, and therefore any correlation with aeromagnetic data is inherently uncertain. However, well-defined relative timing to D2, and the parallel nature of an S1 foliation and regional D1 structures identified in the aeromagnetic data permitted the derivation of a moderately reliable regional model indicating northwest to southeast directed shortening at a deep crustal level during the Musgravian Orogeny. This resulted in a pervasive gneissic foliation, and a regional array of northeast-trending reverse-shear zones and tight to isoclinal upright folds. A higher confidence regional model was derived for the second deformation event (D2), which was identified at the regional scale in outcrop and could be directly correlated with features in the aeromagnetic data. North–south-directed crustal shortening, either in the late Musgravian Orogeny or during deformation ca 1060 Ma resulting in east-southeast- and southeast-trending reverse- and dextral-reverse shear zones and southeast-trending recumbent isoclinal nappes. For the third and fourth deformation events, links could not be made between field observations and aeromagnetic data, and the regional models for these events are low in confidence. The regional setting of D3 is not defined, and D4 is interpreted to represent the development of shear zones during north–south compression in the Petermann Orogeny.

An extensive subglacial lake and canyon system in Princess Elizabeth Land, East Antarctica

The subglacial landscape of Princess Elizabeth Land (PEL) in East Antarctica is poorly known due to a paucity of ice thickness measurements. This is problematic given its importance for understanding ice sheet dynamics and landscape and climate evolution. To address this issue, we describe the topography beneath the ice sheet by assuming that ice surface expressions in satellite imagery relate to large-scale subglacial features. We find evidence that a large, previously undiscovered subglacial drainage network is hidden beneath the ice sheet in PEL. We interpret a discrete feature that is 140 × 20 km in plan form, and multiple narrow sinuous features that extend over a distance of ∼1100 km. We hypothesize that these are tectonically controlled and relate to a large subglacial basin containing a deep-water lake in the interior of PEL linked to a series of long, deep canyons. The presence of 1-km-deep canyons is confirmed at a few localities by radio-echo sounding data, and drainage analysis suggests that these canyons will direct subglacial meltwater to the coast between the Vestfold Hills and the West Ice Shelf.

Did the growth of Tibetan topography control the locus and evolution of Tien Shan mountain building

The Tien Shan in Central Asia are the pre-eminent example of intraplate mountain building, and are typically considered an indirect result of the India-Eurasia collision, via the northward indentation of the relatively strong Precambrian lithosphere of the Tarim plate into the relatively weak Paleozoic lithosphere of the Tien Shan region. This hypothesis fails to fully explain the concentration of Tien Shan mountain building along a Tibet-parallel axis, located 500–700 km from Tibet, and also fails to explain the close synchronization of Tien Shan mountain-building events with mountain building in Tibet. I suggest that bending stresses from lithospheric flexure in Central Asia exerted a critical influence on the locus and evolution of the Tien Shan. Models of Paleogene to Middle Miocene flexure indicate that the growth of northeast Tibet may have superimposed a strong bending stress field on the regional stress field. The resultant stress field is characterized by strong topography-normal tensile stress in the Tarim Basin, and an ∼15% increase in compressive stress in the vicinity of the Tien Shan. This stress field severely reduces the likelihood of mountain building south of the Tien Shan, and increases the likelihood of mountain building within the Tien Shan. This result suggests that the generation of bending stresses from lithospheric flexure is a viable and important mechanism for the propagation of stress, and hence deformation, into continental interiors.

The tectonic development and erosion of the Knox Subglacial Sedimentary Basin, East Antarctica

Sedimentary basins beneath the East Antarctic Ice Sheet (EAIS) have immense potential to inform models of the tectonic evolution of East Antarctica and its ice-sheet. However, even basic characteristics such as thickness and extent are often unknown. Using airborne geophysical data, we resolve the tectonic architecture of the Knox Subglacial Sedimentary Basin in western Wilkes Land. In addition, we apply an erosion restoration model to reconstruct the original basin geometry for which we resolve geometry typical of a transtensional pull-apart basin. The tectonic architecture strongly indicates formation as a consequence of the rifting of India from East Gondwana from ca. 160-130 Ma, and we suggest a spatial link with the western Mentelle Basin offshore Western Australia. The erosion restoration model shows that erosion is confined within the rift margins, suggesting that rift structure has strongly influenced the evolution of the Denman and Scott ice streams.